1. Technical Field
The present disclosure relates to a thermal head which is optimized to a small-sized and thin thermal printer.
2. Related Art
A thermal head mounted on a printing section of a thermal printer is provided with a substrate, a plurality of driver integrated circuits (ICs) which are disposed in the main scanning direction (longitudinal direction) on the substrate, a heater element, and a protective layer which covers the heater element.
The heater element can include a heat storage layer which is made of a glaze glass or the like and extends in the main scanning direction on the substrate; a heating resistor layer which has a plurality of pairs of effective heating portions, each pair having a defined dimension (width dimension) of the main scanning direction and a defined dimension (longitudinal dimension) of a sub-scanning direction and a plurality of connection portions, each connecting the pair of effective heating portions at an end thereof in the longitudinal direction on the heat storage layer and constitutes a heating portion, an insulating layer which covers a surface of the heating resistor layer to define a planar size of the heating portion of the heater element; and an electrode layer (electrode) of a wiring pattern which is overlaid on the insulating layer to be able to supply electricity to the heating resistor layer.
The electrode layer is provided with a folded electrode which is connected with the pair of effective heating portions and the connection portion at the end thereof in the sub-scanning direction, a separate electrode which is connected with one effective heating portion of the pair of the effective heating portions at the other end thereof in the sub-scanning direction and connected to a corresponding driver IC, and a common electrode which is connected with the other effective heating portion of the pair of the effective heating portions at the other end thereof in the sub-scanning direction. An example of the above-described conventional thermal head can be found in, for example, Japanese Unexamined Patent Application Publication No. 2006-321093.
In recent years, as a printer is required to be mounted on a portable device to be driven by batteries, and the thermal head of the printer having the above-mentioned configuration also is required to be reduced in size. Accordingly, it is essential that forming areas of the wiring patterns for electrodes through which electricity is supplied to heater elements of the thermal head are narrowed.
In addition, a heating resistance of the thermal head using a battery as a driving source has to be small in order to obtain a sufficient power at a low voltage. However, when the forming area of the wiring pattern for each electrode is narrowed and the heater elements for 128 dots are connected to one driver IC, it is difficult to adjust an oversize (width dimension and length dimension) of the wiring pattern to reduce a wiring resistance. In addition, variation in resistance value occurs among the respective heater elements. Since the variation in resistance value generates density unevenness in printing, it is likely impossible to obtain a good printing result.
As a countermeasure about these problems, a method is also considered in which the heating resistor layer constituting the respective heater elements is formed and then applied with a proper voltage pulse thereon to adjust the resistance value to be reduced as is described in, for example, Japanese Unexamined Patent Application Publication No. 2004-255650. However, such an adjustment has to be performed on the respective heads, and that is very cumbersome. In addition, since the number of the manufacturing steps of the thermal head is increased, manufacturing costs are also increased.
In addition, there is a proposal in which the size of the heating resistor constituting each heater element is changed. However, the dot sizes thereof are different from each other, and distortion occurs in the printing result. Further, energization correction (reverse correction) may be considered to be performed on the heating resistor constituting each heater element, but a correction ratio is changed according to the variation of the thermal head as a product, a printing pattern, or a printing ratio, making it difficult to perform a uniform energization correction.
In addition, the printing portion of the thermal printer heats the heater elements of the thermal head selectively by supplying electricity thereto, and necessarily presses a recording medium with a proper pressure. Therefore, in order to obtain a printing result with a good degree of gloss and image clarity (sharpness of reflection) like a picture on a surface of a recording medium, the surface of the thermal head with which the recording medium comes into contact in printing should be smooth without a step.
Here, on the surface of the protective layer which is formed as an uppermost layer of the thermal head, in particular a step is formed, which is resulted from a thickness of a resistor layer or an electrode layer which are formed on the lower layer thereof. Generally, the step of the resistor layer is formed thin to have the thickness of 0.1 to 0.2 μm, the step of the electrode layer made of aluminum (Al) or the like is formed to have the thickness of 0.7 to 1.0 μm. Therefore, in particular, the step caused by the thickness of the electrode layer much affects the quality of the printing result. Here, in order to remove the step, a working process has been generally implemented to achieve smoothing by polishing the surface of the protective layer as described in, for example, Japanese Unexamined Patent Application Publication No. 2005-224992 and Japanese Unexamined Patent Application Publication No. 2006-335002.
However, a working for removing a step of the surface of a protective film using a polishing operation may include a secondary working, which may increase the number of man-hours. In addition, a load on manufacture, such as variation in the shape of the heater element after removing the step, increases.
In addition, in order to downsize a thermal head and increase a yield of the heater element, a heating resistor may be disposed on an inclined position rather than on the top portion of a heat storage layer formed in a convex shape. Moreover, in manufacturing steps, the surface of the thermal head in the wafer state may be polished in many cases. In such a case, it is very difficult to polish a folded electrode which is disposed on the deepest position (position away from the protruded top portion) in inclination of the convex heat storage layer while keeping its curvature. Therefore, a polishing process becomes easier as the dimension of the folded electrode is shorter. However, if the dimension of the folded electrode is too short, a heat distribution of the heating resistor required for printing is not accomplished. For this reason, if the folded electrode excessively accumulates heat, an ink ribbon may be affected by damage (thermal damage) when the ink ribbon is detached, which adversely affects the ink ribbon to get torn, wrinkle, or the like.
These and other drawbacks exist.